Old Pipe Spec

[DSB123]

Hello Members,
Would like your responses to the following:-

Consider an existing Pipe Spec for a plant which was developed in 1992 to cover A335 P11 pipework to ASME B31.1

The selected pipe schedules were Sched 120 which were determined to be acceptable for the design conditions at the time according the the wall thickness calcs. However if you check out the required wall thickness to the 1998 or later editions of the Codes then Sched 120 is not thick enough. The reason being the reduced allowable stress in the later Codes ( 9300psi vs 11000psi originally used)

My concern is that the original Pipe Spec cannot be used for any changes to piping under that pipe spec and a new pipe spec must be developed for the plant for any modifications/extensions which were performed from when the allowable stress was reduced in the Code. Is this approach correct?

Regards

 

[JohnBreen]

Hello DSB123,

Yes. We found many piping failures in the field with 1 1/4 Cr, 1/2 Mo material, especially at branch connections, due to creep-fatigue interaction and so we recalculated the allowable stress (at temperature) curve for the next Code update.

With that understanding it would not be good practice to use the old allowable stresses from the original Code of record. Modifications, repairs and replacements should incorporate the new data or simply go to 2 1/4 Cr, 1 Mo material as the replacement.

The above is my opinion only and does not necessarily reflect the opinion of any Code body or its sponsor.

Regards, John.

 

[DSB123]

Thanks John for the quick response. Your thoughts concur with my own. I have already advised the plant that the original pipe spec should not be used but it's the "old" response that you get "well the piping has been in and operating since 1992 so it must be OK". It's difficult to instigate changes when you get such a response. They probably will only address the situation when there is a failure.

Again many thanks for your "solid" response.

Regards

DSB123

 

[sjrfc2]

From a practical approach you may want to look at 2-1/4 (P22) material as a replacement. When we looked at using the new stress values for an existing plant we were buying into thicker piping and driving the need for a revised flexibility analysis. Frequently the original flexibility analysis wasn't available so it created the prospect of doing a lot of work. We found we could use P22 at the original design wall w/o driving further analysis.

From what I was able to find a lot of documented failures were at welds and HAZs (notably the failure of seam welded piping). We had smaller diameter seamless piping in our plants so I was never sure we were in the same risk category as the seam piping. We adopted the lower stress values though.